In recent years, batteries such as lithium ion secondary batteries are used in various fields, for example, electronic devices such as a mobile phone and a personal computer, vehicles such as a hybrid vehicle and an electric vehicle. In particular, the lithium ion secondary batteries provide high energy density and thus are suitably mounted in various devices.
The lithium ion secondary battery is for example configured such that a power generating element is housed in a rectangular battery case. This power generating element is for example formed in a flat wound shape including a positive electrode sheet having positive coated layers containing positive active material, a negative electrode sheet having negative coated layers containing negative active material, and separators insulating them. The battery case is provided with a case body having an opening on an upper side to house the power generating element, and a case lid closing the opening of the case body.
The case lid is fitted in the opening of the case body and welded thereto. This welding is performed for example by laser welding using a CW laser in such a manner that a laser beam is irradiated from above the battery case to an upper surface thereof by vertical-shooting case-sealing welding. A welded zone in the battery case is a boundary appearing on the upper surface of the battery case between the case body and the case lid. This boundary is an annular area formed inside the peripheral edge of the upper surface of the battery case. When the CW laser is irradiated to the boundary, a plume rises up, or blows out, from the boundary in a nearly vertical direction. The plume is a vaporized metal rising like smoke or fume and mainly composed of Ar (Argon) vapor and plasma. During laser welding, a shield gas is made to flow along the boundary in order to prevent the plume from greatly fluctuating or deflecting in an inside-outside direction of the battery case, that is, in order to allow the plume to stably rise up from the battery case in the nearly vertical direction.
Herein, the case lid is attached with electrode terminal members (a positive terminal member and a negative terminal member) electrically connected to the power generating element. The positive terminal member connected to the positive electrode sheet of the power generating element is identical in structure to the negative terminal member connected to the negative electrode sheet of the power generating element. The electrode terminal members each include an element connecting terminal (a positive connecting terminal, a negative connecting terminal) and an outer connecting terminal. Each element connecting terminal (a positive connecting terminal, a negative connecting terminal) has an insert-through part inserted through a through hole formed in the case lid and is electrically connected to the power generating element. Each outer connecting terminal is for example a Z terminal of a nearly Z-shape in side view in a longitudinal direction of the rectangular battery case (hereinafter, simply referred to as a “longitudinal direction”) and is electrically connected to the element connecting terminal outside the case lid. Insulators are provided between each outer connecting terminal and the case lid. The insulators are insulating members made of synthetic resin and used to insulate the outer connecting terminals from the case lid. Furthermore, gaskets are provided between each outer connecting terminal and the case lid. The gaskets are used to seal a gap between each element connecting terminal and the case lid and also to insulate between them. Assembling the electrode terminal members and others to the case lid is performed by sequentially inserting the gaskets, the case lid, the insulators, and the outer connecting terminals onto the corresponding insert-through parts of the element connecting terminals and then deforming, or riveting, a distal end portion of each insert-through part.
The width of the insulator provided in the battery (the width in a short side direction of the rectangular battery case) is slightly smaller than the width of the rectangular battery case itself in the short side direction. Accordingly, the separation distance from the outer peripheral surface of each insulator in a longitudinal direction to the boundary portion of the battery case is short. Thus, a flow path of a shield gas G is small in vertical cross section as shown in FIG. 17, resulting in an insufficient flow along a boundary portion K.
In some cases, therefore, a plume F would largely fluctuate in the inside-outside direction of a battery case 110. If the plume F largely fluctuates toward the inside of the battery case 110, the high-temperature plume F may touch and hence burn an insulator 180. If the insulator 180 is burned, the insulation property of the insulator 180 deteriorates, causing a defect that could not insulate between an outer connecting terminal 137 and a case lid 113.
Meanwhile, there is known a battery disclosed in Patent Document 1 listed below as a battery including a case body and a case lid welded to each other by a laser beam irradiated from above a battery case toward an upper surface thereof. In the battery disclosed in Patent Document 1 listed below, as shown in FIGS. 1 and 2 and paragraphs 0030 to 0032, a groove 311 is formed along the outer periphery of a case lid (a sealing plate 31), thereby forming a stepped part 312 in the inner surface of the groove 311, the stepped part 312 having a height lower in a section close to the center than in a section close to the outer periphery. The presence of this stepped part 312 eliminates a direct heat transfer path from a molten pool 60 toward the center of the sealing plate 31 along the surface of the sealing plate 31. Accordingly, it is possible to suppress dissipation of heat from the molten pool 60 to the center of the sealing plate 31 during laser sealing, thereby reducing thermal damages to an insulating member (a gasket 33) and other members located around a weld zone.